This research is directed to identifying the mechanism underlying the correlation of late timing of puberty with high spatial ability. Since males reach puberty later than females and also have higher spatial ability, this research is relevant also to the causes of sex differences in spatial ability. D. Waber (1977a, b) has proposed that brain lateralization is more pronounced in late maturers, and that this is the cause of their higher spatial ability. An alternative theory is that differences between early and late maturers in personality and activities causes the spatial ability difference. The project will involve a longitudinal study of males and females, studied at age 13 (eighth grade) and again at 15 (tenth grade). Additional subjects will be studied to control for possible effects of repeated testing, and for time of testing/cohort effects. Dropout effects will also be assessed. Five categories of variables will be measured: timing of puberty (4 measures) cognitive ability (spatial and verbal, 2 measures each), lateralization (for both spatial and verbal stimuli, using 2 tachistoscopic tasks), personality (4 measures) and activity (3 measures). Handedness will also be assessed. Multiple regression will be used to evaluate the alternative causal hypotheses. The understanding gained of the determinants of individual differences and sex differences in spatial ability should help in deciding how to best train and foster spatial ability, which seems to aid success in a variety of important occupations, such as mathematics and engineering.